1. Field of the Invention
The invention relates generally to a method and apparatus for restoring a workpiece support surface in a semiconductor wafer processing system to a pre-process condition and, more specifically, to reducing the accumulated charges on the workpiece support surface.
2. Description of the Background Art
Electrostatic chucks are used for retaining a workpiece in a variety of applications including securing a substrate (i.e., a semiconductor wafer) within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for securing or clamping wafers to a pedestal located within the process chamber. The pedestal is provided with additional equipment, e.g., heaters, heat sinks, heat transfer gas ports, additional electrodes and the like to regulate temperature, electrical bias and other chamber conditions during wafer processing to optimize yield. In certain types of chambers, for example an inductively coupled plasma source (IPS) sputter etch chamber, a plasma is used to initiate wafer processing conditions. In such chambers, the pedestal also functions as a RF powered cathode. An RF powered anode is typically the chamber walls. An additional coil on the outside surface of the chamber lid is energized with RF power that inductively couples through the lid (the lid being a dielectric, such as quartz) and into the chamber. The electric field generated between the anode and cathode along with the inductively coupled power from coil ionizes a reactant gas introduced into the chamber to produce the plasma. The plasma, characterized by a visible glow, is a mixture of positive and negative reactant gas ions, neutrals and electrons. Ions from the plasma bombard the wafer to create (etch) a desired pattern.
Electrically biasing the pedestal and wafer as a cathode enhances the wafer process; however, it also creates certain undesirable conditions afterwards. Particularly, charges accumulate on the dielectric support surface of the electrostatic chuck. The source of these charges is primarily ionization of a backside gas. Backside gas (or heat transfer gas) is pumped through the pedestal to heat transfer gas ports in the support surface to be maintained between the support and the wafer. This gas transfers heat between the wafer and the support to maintain uniform temperature conditions across the entire backside of the wafer as it is clamped to the support surface. However, if the backside gas becomes ionized, some of the electrons are attracted to and ultimately accumulate on the support surface (e.g., the electrostatic chuck) due to the bias on the pedestal. Other sources of accumulated charges are stray ions from the plasma that are erroneously deposited on the edges of the electrostatic chuck or the field emission effect which pulls charges from the backside of the wafer to the support surface.
Accumulated charges are detrimental because they reduce the available chucking force for retaining a wafer on the support surface. This condition, in turn, results in poor process conditions. For example, a reduced chucking force can contribute to a non-uniform backside gas pressure under the wafer. Such unequal forces cause wafer shifting or pop-off and compromise temperature control which results in poor etch process conditions or particle contamination. Additionally, during the course of batch processing, it becomes increasingly difficult to dechuck a processed wafer due to the buildup of accumulated electrostatic charges in the support surface. As such, it is necessary to provide methods for eliminating these accumulated charges.
Various methods for removing charges from support surfaces are available and known in the prior art. The simplest method is to switch off the power to chucking electrodes, and/or ground the chucking electrodes. However, this method is also the least effective because the residual charges do not conduct through the dielectric to the electrodes. Instead, these residual charges remain on the support surface. Mechanical wiping is a second method of charge removal. However, this solution requires additional machinery be incorporated into the process chamber which can be costly and increase the frequency of maintenance cycles for repairs, adjustments and the like.
Plasma post-processing the chuck surface is a recognized solution and involves exposing the support surface to a highly energetic plasma after the wafer has been removed. The mechanism by which the plasma works though is not totally understood. One school of thought is that the plasma provides an electrical return path for the accumulated charges to drain. Yet another school of thought is that the plasma physically bombards the support surface to loosen the accumulated charges. Either way, the plasma removes accumulated charges from the support surface. Invariably, the plasma also attacks and deteriorates the material of which the support surface is made. Prolonged or repeated plasma treatment therefore shortens the lifespan of the chuck. Additionally, after repeated bombardment, charges from the plasma can also accumulate on the support surface. Currently practiced methods of surface charge removal do not adequately restore the support surface to a preprocess, i.e., discharged or neutral, condition without damaging or shortening the lifespan of the electrostatic chuck.
Therefore, there is a need in the art for an improved method for removing residual charges from the support surface of an electrostatic chuck that restores the support surface to preprocess conditions without reducing chuck performance or lifespan.